1. Field Of The Invention
This invention relates to a thermally developing apparatus which thermally develops a photosensitive material having an image produced by image-like exposure.
2. Background Art
As an example of an image recording material using microcapsules which contain a photosensitive composition, a system is known which utilizes a synthetic macromolecule resin wall capsule containing a vinyl compound, a photosynthesis initiator and a coloring agent precursor on a support, as disclosed in, for example, Japanese Published Unexamined Patent Application 179836/1982. An image recording method using this system includes the steps of hardening the microcapsules in an image-like manner by exposure. bursting the unhardened microcapsule by pressurization and obtaining a color image from the emitted coloring agent precursor. This method has the feature that it provides a high quality image by simple dry processing. However, this system has the drawback that it has a very low photosensitivity compared to a system using silver halide.
A novel recording material which has a high photosensitivity and provides a high quality image by simple dry processing is disclosed in Japanese Patent Application 117089/1985 filed by this Applicant. This recording material includes a photosensitive composition which comprises at least a photosensitive silver halide, a reducing agent, a polymerizing compound and a color image forming material coated on a support with at least the polymerizing compound and color image forming material being encapsulated.
A method of recording an image using this photosensitive material is disclosed in Japanese Patent Application 121284/1985 filed also by this Applicant. This image recording method includes the steps of first forming a latent image by image-like exposure, performing thermal development, and polymerizing that part of the polymerizing compound where the latent image exists to produce a high molecular compound to thereby harden the microcapsules. It further includes the steps of superposing and pressing together the sheet containing the hardened microcapsules and an image receiving material having an image receiving layer to which the color image forming material can be transferred, bursting at least a portion of that area of the microcapsule sheet where no latent image exists, and transferring the color image forming material to the image receiving material to obtain an image on the image receiving material.
An image recording device which automatically uses the same photosensitive material as that mentioned, performs image-like exposure of the material, thermally develops the material, superposes and pressurizes the resulting material and an image receiving material to thereby transfer an image to the image receiving material is disclosed in, for example, Japanese Patent Application 51896/1986 also filed by this Applicant.
In an image recording device of this type, the thermally developing device is supported by, for example, a heating roller which is heated to about 120.degree. C. and an endless belt supported by a belt support roller and wound around the outer periphery of the heating roller over its circular arc of about 270.degree. whereby a photosensitive material is held and carried between the heating roller and endless belt to thereby be heated.
The photosensitive material is pressed against the heating roller by the tension of the endless belt in order to be heated and developed. Therefore, in order to perform good thermal development, for example, an endless belt 32 cm wide must be stretched at a constant tension of 2-5 kg per width
However, it is difficult to manufacture endless belts having the same peripheral length. An endless belt may be lengthened by heating compared to its length at room temperature. Therefore, the tensions of endless belts used are not constant, but may be too high or too low.
If the tension of an endless belt is too high, the photosensitive material will be too strongly pressed against the heating roller, so that the microcapsules are crushed before they are thermally hardened to thereby provide no proper copied image.
If the tension of the endless belt is too low, the belt is likely to shift laterally on the support roller to slip off from the support roller to render it impossible to heat and carry the photosensitive material. In addition, the pressing force against the heating roller is insufficient, so that part of the photosensitive material floats from the heating roller, the moisture evaporated from the photosensitive material or the support is deposited or collected on the floated portion to thereby expedite the development to produce uneven image quality or wrinkles in the photosensitive material.